Interactions of dietary estrogens with human estrogen receptors and the effect on estrogen receptor-estrogen response element complex formation

Epidemiologic and experimental studies support the hypothesis that dietary estrogens from plant sources (phytoestrogens) may play a role in the prevention of breast and prostate cancer. The molecular mechanisms for such chemopreventive effect are still unclear. We investigated the possibility that phytoestrogens may bind differentially to estrogen receptor proteins (ER alpha and ER beta) and affect the interactions of the ligand-ER complexes with different estrogen response element (ERE) sequences. We used fluorescence polarization to measure the binding affinities of genistein, coumestrol, daidzein, glyceollin, and zearalenone for human ER alpha and ER beta. Competition binding experiments revealed higher affinity of the phytoestrogens for ER beta than for ER alpha. Genistein [median inhibitory concentration 12nM] is the most potent and has the same relative binding affinity for ER beta as 17 beta-estradiol. We also studied the effect of these phytoestrogens on the ability of ER alpha and ER beta to associate with specific DNA sequences (EREs). The direct binding of human recombinant estrogen receptors to fluorescein-labeled EREs indicates that phytoestrogens can cause conformational changes in both human ERs, which results in altered affinities of the complexes for the ERE from the Xenopus vitellogenin A2 gene and an ERE from the human pS2 gene.